Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures

被引:63
作者
Auzende, AL
Guillot, S
Devouard, B
Baronnet, A
机构
[1] CNRS, Ctr Rech Mat Condensee & Nanosci, F-13288 Marseille 09, France
[2] CNRS, UCB ENS Lyon, Lab Sci Terre, F-69622 Villeurbanne, France
[3] CNRS, UBP OPGC, Lab Magmas & Volcans, F-63038 Clermont Ferrand, France
关键词
serpentinite; antigorite; HP-LT metamorphism; microstructures; deformation; pressure solution;
D O I
10.1127/0935-1221/2006/0018-0021
中图分类号
P57 [矿物学];
学科分类号
070901 ;
摘要
Alpine antigorite serpentinites associated with eclogites were investigated to determine if they can be used as indicators of the tectono-metamorphic conditions during subduction and exhumation processes. The detailed petrology of serpentinites sampled in the Monviso massif (Western Alps, Italy) was combined with a transmission electron microscopy study. Alpine serpentinites display a degree of serpentinization close to 100%. Antigorite is the main mineral present, forming non-pseudomorphic textures in the various studied samples and exhibiting a homogeneous chemical composition with limited cationic substitutions. Considering its oceanic origin, the Alpine serpentinite in the Monviso massif formed a lizardite + chrysotile assemblage that recrystallized under greenschist-facies conditions into poorly ordered antigorite, with a modulation wavelength showing significant variations at the crystal scale. Under blueschist-facies conditions, the modulation wavelength of antigorite becomes regular. Thus, periodic antigorites can be related to high-grade conditions, while poorly ordered antigorites characterize lower metamorphic grade. In the present study, we failed to observe any elimination of structural defects with increasing metamorphic grade. While around 50% of the antigorite crystals are highly ordered, it seems that this ordering is at least partly obliterated by retrogressive deformation. Antigorite displays strong evidence of deformation-sensitivity, and the observed microstructures can be directly related to the mechanical behaviour of serpentinites in subduction zones. We investigated the deformation-induced microstructures in serpentinites collected in the Erro-Tobbio eclogitic unit (Ligurian Alps, Italy), which appear to preserve prograde and retrograde structures formed during subduction. According to the microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. Collected samples were fractured at different scales (cm to nm) and have a well-developed schistosity characterized by a strong crystallographic fabric. With increasing metamorphic grade, the brittle behaviour gives way to pressure-solution, which persists up to eclogite-facies conditions. The common obliteration of high-grade microstructures in antigorite, as observed in the Monviso serpentinites, results from continuous recrystallization of this mineral during retrogressive deformation.
引用
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页码:21 / 33
页数:13
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